Bilevel Optimization Benchmark

Results can be consulted on https://benchopt.github.io/results/benchmark_bilevel.html

BenchOpt is a package to simplify and make more transparent and reproducible the comparisons of optimization algorithms. This benchmark is dedicated to solvers for bilevel optimization:

$$\min{x} f(x, z^*(x)) \quad \text{with} \quad z^*(x) = \arg\min_z g(x, z), $$

where $g$ and $f$ are two functions of two variables.

Different problems

This benchmark currently implements two bilevel optimization problems: regularization selection, and hyper data cleaning.

1 - Regularization selection

In this problem, the inner function $g$ is defined by

$$g(x, z) = \frac{1}{n} \sum{i=1}^{n} \ell(d_i; z) + \mathcal{R}(x, z)$$

where $d_1, \dots, d_n$ are training data samples, $z$ are the parameters of the machine learning model, and the loss function $\ell$ measures how well the model parameters $z$ predict the data $d_i$. There is also a regularization $\mathcal{R}$ that is parametrized by the regularization strengths $x$, which aims at promoting a certain structure on the parameters $z$.

The outer function $f$ is defined as the unregularized loss on unseen data

$$f(x, z) = \frac{1}{m} \sum{j=1}^{m} \ell(d'_j; z)$$

where the $d'_1, \dots, d'_m$ are new samples from the same dataset as above.

There are currently two datasets for this regularization selection problem.

Covtype

Homepage : https://archive.ics.uci.edu/dataset/31/covertype

This is a logistic regression problem, where the data is of the form $d_i = (a_i, y_i)$ with $a_i\in\mathbb{R}^p$ are the features and $y_i=\pm1$ is the binary target. For this problem, the loss is $\ell(d_i, z) = \log(1+\exp(-y_i a_i^T z))$, and the regularization is simply given by $$\mathcal{R}(x, z) = \frac12\sum{j=1}^p\exp(x_j)z_j^2,$$ each coefficient in $z$ is independently regularized with the strength $\exp(x_j)$.

Ijcnn1

Homepage : https://www.openml.org/search?type=data&sort=runs&id=1575&status=active

This is a multicalss logistic regression problem, where the data is of the form $d_i = (a_i, y_i)$ with $a_i\in\mathbb{R}^p$ are the features and $y_i\in \{1,\dots, k\}$ is the integer target, with k the number of classes. For this problem, the loss is $\ell(d_i, z) = \text{CrossEntropy}(za_i, y_i)$ where $z$ is now a k x p matrix. The regularization is given by $$\mathcal{R}(x, z) = \frac12\sum{j=1}^k\exp(x_j)\^2,$$ each line in $z$ is independently regularized with the strength $\exp(x_j)$.

2 - Hyper data cleaning

This problem was first introduced by [Fra2017] . In this problem, the data is the MNIST dataset. The training set has been corrupted: with a probability $p$, the label of the image $y\in\{1,\dots,10\}$ is replaced by another random label between 1 and 10. We do not know beforehand which data has been corrupted. We have a clean testing set, which has not been corrupted. The goal is to fit a model on the corrupted training data that has good performances on the test set. To do so, a set of weights -- one per train sample -- is learned as well as the model parameters. Ideally, we would want a weight of 0 for data that has been corrupted, and a weight of 1 for uncorrupted data. The problem is cast as a bilevel problem with $g$ given by

$$g(x, z) =\frac1n \sum{i=1}^n \sigma(x_i)\ell(d_i, z) + \frac C 2 \^2$$

where the $d_i$ are the corrupted training data, $\ell$ is the loss of a CNN parameterized by $z$, $\sigma$ is a sigmoid function, and C is a small regularization constant. Here the outer variable $x$ is a vector of dimension $n$, and the weight of data $i$ is given by $\sigma(x_i)$. The test function is

$$f(x, z) =\frac1m \sum{j=1}^n \ell(d'_j, z)$$

where the $d_j$ are uncorrupted testing data.

Install

This benchmark can be run using the following commands:

$ pip install -U benchopt
$ git clone https://github.com/benchopt/benchmark_bilevel
$ benchopt run benchmark_bilevel

Apart from the problem, options can be passed to benchopt run, to restrict the benchmarks to some solvers or datasets, e.g.:

$ benchopt run benchmark_bilevel -s solver1 -d dataset2 --max-runs 10 --n-repetitions 10

You can also use config files to setup the benchmark run:

$ benchopt run benchmark_bilevel --config config/X.yml

where X.yml is a config file. See https://benchopt.github.io/index.html#run-a-benchmark for an example of a config file. This will possibly launch a huge grid search. When available, you can rather use the file X_best_params.yml in order to launch an experiment with a single set of parameters for each solver.

Use benchopt run -h for more details about these options, or visit https://benchopt.github.io/api.html.

Cite

If you use this benchmark in your research project, please cite the following paper:

@inproceedings{saba,
   title = {A Framework for Bilevel Optimization That Enables Stochastic and Global Variance Reduction Algorithms},
   booktitle = {Advances in {{Neural Information Processing Systems}} ({{NeurIPS}})},
   author = {Dagr{\'e}ou, Mathieu and Ablin, Pierre and Vaiter, Samuel and Moreau, Thomas},
   year = {2022}
}

References

Fra2017

Franceschi, Luca, et al. "Forward and reverse gradient-based hyperparameter optimization." International Conference on Machine Learning. PMLR, 2017.